1. Field of the Invention
The present invention relates to an echo canceller for bidirectional transmission on two-wire subscriber lines utilizing metallic subscriber lines in an integrated service digital network (ISDN).
2. Description of the Prior Art
In recent years, a number of field trials for the ISDN, which adopts a bidirectional transmission system with an echo canceller utilizing existing metallic subscriber lines for interfacing with subscribers, have been conducted in many countries.
Such an echo canceller circuit is usually coupled to subscriber lines through transformers to exchange transmission and reception signals. For a so-called echo signal, which is part of the transmission signal reflected into the receiver section of its own circuit, the usual method is to achieve a certain degree of attenuation with a hybrid circuit having a simple balancing circuit, and to completely remove further the echo signal at a suppression level of about 60 decibels (dB) or more with an echo canceller which generates and subtracts an echo replica with an adaptive filter having transmission symbols as its input. At this time, the required number of taps of the transversal filter is determined by the length of the inpulse response of the echo entered into the echo canceller section, so that an echo path equalizing filter to shorten this impulse response length is interposed somewhere between the transmission driver and the echo canceller section. Since the long tailing-off part of an impulse response (echo tail) contains many low frequency components, conventionally such a high-pass filter as will suppress the low frequency components is used as this echo path equalizing filter. Usually a fully D.C.-intercepting type high-pass filter is used to make the direct current (D.C.) loss infintely great. A similar arrangement is described by P. F. Adams et al in a paper entitled "Long reach duplex transmission systems for ISDN access" published in the Br Telecom Technol J. Vol. 2, No. 2, April issue, 1984, pp. 35-42.
However, when the recently proposed 2B1Q line codes represented by a random sequence with a D.C. spectrum are applied to an echo canceller provided with the above-mentioned high-pass type filter, the echo canceller generates an unremoved echo residual corresponding to the result of convolution of an echo impulse response component left afteer the (N+1)th tap on the time axis. For instance, if the tap number N is 30 or so and an inductance value is about 50 mH, the average power of this echo residual will be about -10 dB in the absence of echo path equalizing filter, or about -40 dB where an echo path equalizer filter of a full D.C. intercepting type is used. These values are much less than the usually required suppression level of -60 dB. Meanwhile, though it is conceivable to achieve an adequate level of suppression by increasing the tap number N, the tap number N of the echo path equalizing filter then would have to be 100 or more, which is unrealistic.
An object of the present invention is, therefore, to provide an echo canceller for eliminating a pole having a long time constant and thereby accelerating the attenuation of the echo tail by cancelling the pole of an echo path transfer function attributable to the inductance component of the line coupling transformer.
Another object of the invention is to provide an echo canceller capable of reducing the number of taps of a filter by cancelling the pole of said echo path transfer function.
In order to achieve the foregoing objects, in an echo canceller according to the present invention, there is arranged within the echo path a filter having such a zero point as will cancel the pole of the echo path transfer function attributable to the inductance component of the line coupling transformer.